1. Field of the Invention
The present invention relates to a tandem electrical control and, more particularly, to a pair of controls connected in tandem with concentric shafts.
In such tandem controls, the rotatable contactor assembly of the unit closest to the panel is usually driven by a tubular shaft or sleeve, while the other unit is independently driven by a solid shaft disposed inside and projecting from the tubular shaft. There are many instances when a switch secured to the rear unit is also driven by the solid shaft. The switch is actuated at some point of rotation of the solid shaft and often is used to turn on or off the apparatus in which the control has been installed.
2. Description of the Prior Art
A binding or seizing defect in the control is a common problem encountered by manufacturers and repairmen. It has been found that considerable damage occurs when one of the shafts is accidentally struck and the impact forces are transmitted to the corresponding variable resistance unit. In the past, various means have been devised for eliminating impact damage. U.S. Pat. No. 3,389,364, of common assignee, describes the use of a "C" washer on a single shaft control. U.S. Pat. No. 3,747,043, also of common assignee, describes a "C" washer engaging a groove in the wall of the outer tubular shaft and a groove in the inner shaft, and abuttingly engaging the distal end of a bushing to transfer to the bushing impact forces applied to either shaft. It has been found that the "C" washer construction is not entirely satisfactory. The "C" washer is easily deformable and presents a small area of engagement with the bushing and with the grooves of each of the shafts. A small deformation of the "C" washer can destroy the feel of the control thereby making the control unusable even though not binding. Additionally, if a switch is used on the control, a small deformation of the "C" washer can make the switch nonfunctional. If the strength of the "C" washer is increased, as by increasing the thickness or hardness of the material, then upon impact of the shaft, the shaft can be deformed and driven into the "C" washer because of the small bearing area between the shaft and the "C" washer and the high pressures thereby generated.
The functional equivalent of a "C" washer may be formed by an abrupt shoulder integrally provided on the shaft in abutting engagement with the distal end of the bushing. The integral shoulder is a distinct improvement over the "C" washer construction but suffers from the same aforementioned difficulty; namely, that of a small bearing area between the shoulder and the bushing. Additionally, a shaft with an abrupt or 90.degree. shoulder is not self-centering within the outer member. If the shaft is disposed in an off-centered manner when the control sustains axial impact forces to the shaft, any deformation of the shaft or of the impact protection bearing surfaces caused by such forces, will be off-centered or nonconcentric with respect to the shaft axis. The off-center deformation may sufficiently damage the feel of the control or force the shaft to be sufficiently permanently off-center so as to make the control nonfunctional. In a self-centering construction, impact damage will cause a concentric seating of the two members and maintain concentricity and feel. It is therefore desirable to provide a control wherein the shafts are impact protected and such impact construction is self-centering.
It is often desirable to enable the user to adjust the control to a preselected position by feel rather than by sight as with car radios where it is desirable to enable the user to set a tone control for approximately flat tonal response or to select preselected degrees of tonal boost or cut without requiring the driver to look away from the road. Another such situation may arise in an attenuator or volume control where the control setting is determined by a detent feel rather than by sight. In such detented situations, it is often desirable to permit the user to set the control at any intermediate position between detent settings thereby providing the user with a continuously variable control capable of being set intermediate the discrete digital or quantized detent settings. The prior art as exemplified by U.S. Pat. Nos. 2,632,830 and 3,832,671 does not permit such nondetent settings of the control.
Additionally, positional detents used in variable resistance controls are often provided by a protrusion in the rear portion of the housing axially engaging the driver with the driver being provided with a recess adapted to at least partially receive the housing protrusion. The driver rides up on the protrusion between detent positions causing axial movement of the corresponding control shaft. This axial movement of the control shaft changes the tracking pressure or contacting pressure of the contactor member thereby tending toward erratic resistance settings. In addition, if a knob is secured to the shaft in proximity to a mounting panel, the knob can be driven into abutting interference engagement with the mounting panel by such axial movement in the detent position thereby binding or hindering further rotation of the control.
In response to the demand for miniaturization, it is desirable that controls be made smaller. In the past, the collector ring or member in contact with the rotatable contactor member has been spaced apart from the resistance element by legs extending from the outer periphery of the ring, thereby causing the collector ring to be disposed inwardly from the legs toward the shaft, and thereby restricting the shaft diameter. As the controls are made smaller, it is necessary that the portion of the shaft internal to the control be made thinner thereby weakening the shaft; whereas it is desirable to maximize the shaft diameter inside the housing as a part of the total program of miniaturization, it is essential to maintain a suitable diameter of the shaft inside the control housing in order to maintain the strength of the shaft, particularly for plastic shafts.
A common problem encountered in controls is the deviation from an ideal tracking path due to parts tolerances and malformed parts. Such deviant or poor tracking can result in an uneven wiping pressure between the contactor member and the resistance element or the collector member, causing a bad feel to the control and an increase in electrical noise manifesting itself as a nonuniform statistical deviation from the desired smooth resistance change when the control shaft is rotated. In the extreme, the tracking error manifests itself in the contacting portions of the contactor member riding off or disconnecting from the resistance element on the collector member. It is therefore desirable to minimize tracking error effects.